The present invention relates to an electrochromic display device (which is hereinafter sometimes abbreviated ECD).
Compared to liquid crystals, ECD provides easy viewing in the absence of dependency on the angle of view. On the other hand, ECD does not have a long life to withstand repeated use and it is difficult to produce more than one color with ECD. Because of these disadvantages, efforts to commercialize ECD have met with only limited success.
It was already proposed that phthalocyanine complexes of metals be used as electrode materials to make ECDs capable of displaying in more than one color (see Unexamined Published Japanese Patent Application No. 63-61077). ECDs using metal complexes of phthalocyanine in the display electrode depend for their operation on the phenomenon that the phthalocyanine ring is partially oxidized by an electrochemical redox reaction to cause a color change subsequent to the formation of .pi.-cation radicals. Depending on the redox potential, more than one color change such as blue, green or brown can be produced and this has been impossible with the prior art ECDs.
One important requirement for successful ECD operation is that redox reaction of a magnitude, that is equal to or greater than what occurs in the display electrode, should also be created in the counter electrode. The current practice employed to meet this requirement is to use an electrochromic material in the counter electrode which is the same as what is used in the display electrode but which is thicker than the latter by a factor of about 4. This practice, however, has a problem originating from the use of metal-phthalocyanine complexes; that is, such complexes are stable in a transition from the neutral blue state to a reduced state but they tend to deteriorate (decompose) when a positive voltage is applied to cause a transition from the neutral state (V=0) to an oxidized state. The present inventors therefore thought of extending the life of ECDs by making both a display and a counter electrode from a metal-phthalocyanine complex in such a way that the membrane at the counter electrode side is 40 times as thick as the membrane at the display electrode side. This method was effective in extending the device life to some extent but since deterioration does proceed in the metal-phthalocyanine complex in the counter electrode, there still remains the problems that the structure of the complex is slowly destroyed until it becomes no longer capable of producing a reversible color change. This problem is not limited to metal-phthalocyanine complexes and is found in other materials such as phenanthroline and bipyridine. It is therefore desired to develop an ECD with which it is guaranteed that the device can be driven, without deterioration, by as many times as are required to insure reasonable performance in commercial applications.